The present invention is a device for separating semiconductor grade silicon pieces into desired size ranges. The described invention employs a rotating cylindrical screen with all contact surfaces of semiconductor grade silicon. Therefore, contact contamination of the semiconductor grade silicon pieces is minimized during the separation process.
High density, integrated, circuits require wafers of monocrystalline silicon of high purity. Of particular problem, are transitional metal impurities including among others, copper, gold, iron, cobalt, nickel, chromium, tantalum, zinc, and tungsten, and impurities such as carbon, boron, and phosphorous. These impurities, even in small quantities, introduce defect sites in semiconductor material which can ultimately result in degraded device performance, and limit circuit density.
Typically, a polycrystalline silicon of high purity is formed by chemical vapor deposition of a high purity chlorosilane gas onto a heated substrate. The resulting product is rods of polycrystalline silicon. The polycrystalline silicon must be further processed to produce a monocrystalline silicon from which silicon wafers can be cut.
A significant portion of the monocrystalline silicon required by the semiconductor industry is produced by the well known process first described by Czochralski. In a typical Czochralski type process, silicon pieces are melted in an appropriate vessel and a silicon seed crystal is used to draw a monocrystalline rod of semiconductor grade silicon from the melt. Control of this crystal growth process requires that the silicon pieces added to the melt containing vessel be within a defined size range. Therefore, it is necessary that the polycrystalline rods formed during the chemical vaporization deposition process be broken into pieces, and that these pieces be sorted into appropriate size distributions.
The inventors have recognized that screening apparatuses constructed of conventional materials, such as stainless steel, can be a significant source of surface contamination of the sized pieces. The use of semiconductor grade silicon as a screening surface can minimize this contamination. The present invention provides an efficient and effective device for the mechanized sizing of silicon pieces, with minimum potential for contact contamination of the silicon pieces.
Also, provided is a method of assembling a cylindrical screen of semiconductor grade silicon. The hard and brittle nature of silicon metal makes cutting and machining of silicon metal time consuming, difficult, and expensive. Described, is a method of assembling a cylindrical screen of semiconductor grade silicon which involves little need for expensive cutting and machining. The cylindrical screen can be constructed of parallel bars of semiconductor grade silicon formed from a standard chemical vapor deposition process. The parallel bars can be separated by, for example, simple, small, square spacers of semiconductor grade silicon cut from larger polycrystalline silicon rods. A compression apparatus is used to maintain the position of the parallel rods and these internal spacers. External spacers are positioned along the outside of the cylindrical screen to further define the exclusion characteristics of the screen. This method of assembly of the cylindrical screen avoids the need for extensive machining of silicon metal components.